Development and Operating Results of Low
SO2 to SO3 Conversion Rate Catalyst for
DeNOx Application
By
Isato Morita
Yoshinori Nagai
Dr. Yasuyoshi Kato
Babcock-Hitachi K.K., Japan
Dr.Howard N. Franklin
Hitachi America Ltd., U.S.A.
John Cooper
Indianapolis Power & Light Company,
U.S.A.
Abstract
DeNOx process, using Selective Catalytic Reduction of NOx by ammonia, is now widely
applied to reduce NOx emissions from stationary sources. And, nowadays, hundreds of
DeNOx plants for coal fired flue gas have been operating successfully in Japan, Europe,
U.S.A., Canada and Eastern Asian counties. Though, through the operation with very high
Sulfur content coal, the development of low SO2 to SO3 conversion rate catalyst became one
of the most important research works for the catalyst manufacturer. After extensive research
work, we, Babcock-Hitachi developed the low SO2 to SO3 conversion rate catalyst keeping
the DeNOx activity high, and applied it for commercial application, and two DeNOx plants
were successfully got into commercial operation in 2004.
1. Introduction
About 30 years have passed since the first catalytic DeNOx system for large utility boilers
was started operation in 1977 in Japan. Since then, the system was getting more and more
widely applied for various fuels such as gas, oil and coal, and the installation was expanded to
various industrialized countries, Japan, Europe, U.S.A., Canada and Eastern Asian countries.
And nowadays, hundreds of DeNOx plants for coal fired flue gas have been operated
successfully.
Though recently, through the increasing commercial plants’ operation in U.S.A. firing Eastern
Bituminous coal which contains high Sulfur(3 – 5%), several plants got difficulties in
operation, such as severer blue plum generation and Sulfuric acid smut emission. The
increased SO3 concentration also causes (1) acceleration of corrosion on downstream
equipments, (2) increased possibility of Ammonium Bisulfate formation within air preheater
elements and (3) increased PM emission, etc.
To overcome this situation, various research efforts to reduce SO3 emission were conducted
from various aspects such as additives injection to combine SO3 into less troublesome forms
and so on. Though, as a fact of matter, the development of low SO2 to SO3 conversion rate
catalyst can be the most effective solution to minimize the production of SO3 through catalyst,
and therefore it became the most important research target amongst others for the catalyst
manufacturer.
However, the SO2 to SO3 conversion rate was closely related with DeNOx activity. And
when the SO2 to SO3 conversion activity was reduced, larger volume of catalyst became
usually required to maintain the required DeNOx performance. Therefore, the development
has to be focused to find out the catalyst having improved DeNOx activity with low SO2 to
SO3 conversion activity. Through diligent research work, not only for the catalyst
manufacturing process but for the screening of the new materials, we, Babcock-Hitachi(BHK),
has developed low SO2 to SO3 conversion rate catalyst, keeping the DeNOx activity high,
and applied it for commercial application, and two DeNOx plants were already got into
commercial operation in 2004 successfully.
This paper presents this new SO2 to SO3 conversion rate catalyst, including the development
efforts and the results of commercial operation experience.
2. Development of Low SO2 to SO3 Conversion Rate Catalyst
(1) Target Performance for Developing Low SO2 to SO3 Conversion Catalyst
Table 1 gives the representative design gas conditions for DeNOx catalyst for high Sulfur coal
fired flue gas in U.S.A. To dates, the DeNOx plants were usually designed with SO2 to SO3
conversion rate of less than 1.0 to 1.5 %. Though, because of the high NOx removal
requirement (more than 90%) with less than 2 ppm NH3 slip, difficulty appeared to achieve
the requirements at elevated temperature such as 400 to 420 °C. Furthermore, operational
difficulties such as blue plum generation and acid smut emission became appeared in several
DeNOx plants. Then, we, BHK, decided to develop the low SO2 to SO3 conversion rate
catalyst that can achieve less than 0.5 % SO2 to SO3 conversion even at elevated temperature.
Thus, the developed catalyst should have approximately 1/4 of SO2 to SO3 conversion rate
maintaining comparable DeNOx activity as of the catalyst used at the timeframe.
Table 1
Representative Design Conditions for DeNOx Catalyst in U.S. High Sulfur Plants
Items
Condition
Inlet NOx concentration
300 ppm
SO2 concentration
1,800 – 3,500 ppm
DeNOx Effeiciency
More than 90 %
Slip NH3 concentration
Less than 2 ppm
Life time
More than 24,000 hours
(2) Concept for development of low SO2 to SO3 conversion rate catalyst
Continuous efforts were paid to improve the catalyst properties for high dust coal fired
application maintaining its durability since the very early stage. Through the efforts, the
DeNOx catalyst applied for commercial use in early 2000th gave us approximately 20%
higher DeNOx activity compared with the one used in late 1980th, maintaining the similar
SO2 to SO3 conversion activity. Such improvement was become available through the
modification of catalyst manufacturing process and the utilization of new materials. Thus,
further improvement in 2000th was getting more difficult than early dates.
Usually, changing the active component’s composition made the control of catalyst activity.
However, with this technology, the SO2 to SO3 conversion activity was closely dependent on
catalyst active component’s composition. If we add the active components to increase DeNOx
activity, the SO2 to SO3 conversion activity was also increased as shown in Fig. 1. Thus, to
achieve low SO2 to SO3 conversion rate in design, considerable increase in catalyst volume
could not be avoided with the conventional technology.
At this time, we decided to look for the combination of the following process to achieve the
low SO2 to SO3 conversion rate keeping similar or rather higher DeNOx activity compared
with the catalyst applied in 2000th as also shown in Fig. 1.
(a) Investigate the applicability of the new catalyst materials, including the modification
of catalyst manufacturing process suitable for the materials
(b) To prepare the catalyst with thinner thickness maintaining the durability of catalyst
against catalyst poisons
Developed
High
(b)
DeNOx Activity
(a)
Decrease
Low
Low
Fig. 1
Catalyst
Active Components
Increase
Conventional
Technology
High
SO2 to SO3 Conversion Activity
Concept for Development of Low SO2 to SO3 Conversion Rate Catalyst
(3) Results of Development
Apart from the catalyst manufacturer’s efforts to improve the catalyst performance, material
suppliers also made continuous efforts to improve the material characteristics so that the users
could create better final products. By maintaining contacts with material suppliers, several
materials were selected for investigation.
As an example, Fig. 2 shows one of the investigation results to find suitable Titanium Dioxide
material. As shown in Fig. 2, several Titanium Dioxide materials make it possible to reduce
the SO2 to SO3 conversion rate. Similar investigation was also conducted for other catalyst
materials.
DeNOx Eff.
SO2 to SO3 Conversion
1.4
Activity Ratio (-)
1.2
1
0.8
0.6
0.4
0.2
0
Base
A
B
C
D
Material
Fig. 2
TiO2 Material Screening Test Results(Example)
The other issue required for developing new catalyst is, of course, the optimization for the
manufacturing process. As the new materials are usually manufactured with different process,
the physical and chemical characteristics of the materials became different from the
previously used materials. Such change used to cause different mixing characteristics.
Furthermore, by changing the mixing process, the catalyst characteristics are also controlled
to a certain degree.
Through extensive research work, as stated above, we, BHK, developed a new series of
DeNOx catalyst (called CX Type) applied for coal fired flue gases with high dust loading. Fig.
3 shows the catalyst performance of the CX Type catalysts at 350 °C compared with the most
commonly used catalyst (called Base) at the time. In the figure, the performance for the
catalyst used in late ‘80th when DeNOx installation in Europe was started was also indicated.
As observed in Fig. 3, CX and CXL catalyst achieved considerably lower SO2 to SO3
conversion rate maintaining similar DeNOx activity compared with Base catalyst.
DeNOx Efficiency
SO2 to SO3 Conversion
1.4
Activity ratio (-)
1.2
1
0.8
0.6
0.4
0.2
0
'80th
Base
CXL
CX
CXM
Catalyst Type
Fig. 3
Comparison of Catalyst Activity of Developed Catalyst
The durability of the developed catalyst was also tested prior to the commercial application.
The tests were conducted not only in laboratory but also in field. In laboratory, the catalyst
was tested with severer conditions than commercially applied conditions for catalyst poisons
such as Alkaline materials and Arsenic, and also for thermal stability.
Fig. 4 shows the Arsenic resistance test results of the developed catalyst. As U.S. Eastern
Bituminous coal used to contain rather high Arsenic in coal, Arsenic resistance is one of the
most important characteristics required for the catalyst. As shown in Fig. 4, the developed
CX-Type catalyst gave better durability against Arsenic oxide compared with the Base
catalyst.
(K/K0)CX-Type/(K/K0)Base (-)
1.2
1
0.8
0.6
0.4
0.2
0
Base
CXL
CX
CXM
Catalyst Type
Fig. 4
Arsenic Resistance Test Results
Thermal sintering also could decreased the catalyst activity. Therefore, the sintering
characteristic of the CX Type catalyst was tested by exposing the catalyst at 600 °C for the
period of 2,000 hours. Fig. 5 shows the crystal diameter growth rate after 2,000 hours.
Compared with the Base catalyst, the CX Type catalyst gives considerably stable
Crystal Diameter Growth Rate (-)
characteristic for thermal sintering.
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
Base
CX Type
Catalyst Type
Fig. 5
Thermal Stability test Results
The durability of CX Type catalyst was also tested in the field installing the samples into the
flue gas stream from coal-fired plant. Through the field tests, the durability of CX Type
catalyst was also confirmed to be as same as Base catalyst.
All other durability test results for CX Type catalyst gave us better or similar tendency over
the Base catalyst.
After completion of the development, feasibility study was conducted using the conditions
described in Table 1. The results were summarized in Table 2.
As obvious from Table 2, the CX Type catalysts can be used even at an elevated temperature
like 415°C with less than 0.5 % SO2 to SO3 conversion rate. Thus, the CX Type catalyst can
be used more flexibly, and can minimize the impact of SO3 towards the downstream
equipments.
Table 2
Applicable Temperature Ranges for New Installation
Catalyst
Type
SO2 to SO3 Conversion Rate (%)
0.5
1.0
1.5
< 370 °C
< 380 °C
< 390 °C
CXM
< 340 °C
CX
< 380 °C
< 415 °C
< 435 °C
CXL
< 415 °C
< 445 °C
-
Base
< 395 °C
3. Operational results of Low SO2 to SO3 Conversion Rate Catalyst
(1) Design conditions for operating plants with low SO2 to SO3 conversion rate catalyst
The first application of the developed low SO2 to SO3 conversion rate catalyst (CXL) was in
the Indianapolis Power & Light Company’s Petersburg DeNOx units. In Petersburg station,
there are three coal-fired boilers, and the boilers for Units 2 and 3 were retrofitted with
DeNOx plants to reduce NOx emissions from the plants.
Fig. 6 shows the photograph of Petersburg station.
Fig. 6
Indianapolis Power & Light Company / Petersburg Station
A major technical concern for the installation of DeNOx plants was the formation of SO3 by
DeNOx catalyst because of the reports stating the problems on SO3 emission during the
planning stage.
Petersburg station utilizes several local high sulfur eastern bituminous coals. These coal
analysis data was summarized as shown in Table 3.
Table 4 shows the design basis of DeNOx catalyst for Petersburg station.
As shown in Table 4, the application also required higher DeNOx efficiency of more than
90% together with lower slip NH3 of less than 2 ppm compared with the European plants.
Because of these requirements, the DeNOx plants have to be designed to achieve very good
distribution for NH3/NOx, Temperature and gas flow rate. The DeNOx system supplier,
therefore, conducted physical flow model testing to achieve the following inlet variations.
NH3/NOx
Less than ±5 % RMS
Temperature
Less than ±11 °C
Gas flow rate
Less than ±15 % RMS
The final arrangement of Petersburg station Unit No.2 & 3 DeNOx plants were shown in Fig.
7, and Fig. 8. As shown in Figures, two static mixers were installed at the downstream of
ammonia injection grid to achieve the required NH3/NOx distribution criteria for both of the
plants.
Table 3
Coal Analysis Data for Petersburg Boilers
Ultimate Analysis (Dry)
Items
Ash Mineral Analysis
Typical (%)
Items
Range (%; as ash)
Moisture
--
SiO2
34.89 – 56.0
Ash
9.0
Al2O3
16.13 – 26.21
Carbon
73.6
TiO2
0.68 – 0.95
Hydrogen
5.0
Fe2O3
10.92 – 15.60
Oxygen
7.6
CaO
1.47 – 2.48
Nitrogen
1.3
MgO
0.26 – 1.35
Sulfur
3.5
Na2O
0.22 – 1.27
K2O
0.75 – 3.24
SO3
0.2 – 5.25
P2O5
0.02 – 0.78
As in coal (ppm)
5.74 – 14.40
Table 4
Items
Design Basis of DeNOx Catalyst for Petersburg Station
Units
Petersburg #2
Petersburg #3
Number/Boiler
2
2
MW
463
563
Flue Gas Flow
m3/h(Normal)
1,552,065
1,878,943
Flue Gas Temp.
°C
414.5
394.1
%, Dry
4.59
4.65
H2O
% (Volume)
8.78
8.96
SO2
ppmvd @ 3% O2
2,645
2,225
SO3
ppmvd @ 3% O2
25
21
NOx
ppmvd @ 3% O2
303.1
335.6
Dust
mg/m3(Normal)
9,800
10,200
%
More than 90
More than 90
ppmvd @ 3% O2
Less than 2
Less than 2
%
Less than 0.5
Less than 0.5
hours
24,000
24,000
No of Reactor
Generation Capacity
Inlet
O2
DeNOx Efficiency
Slip NH3
SO2 to SO3 conv.
Catalyst Life
Ash cleaning devices were usually installed for the DeNOx plants with high dust coal fired
applications. For DeNOx plants at Petersburg station, sonic horns were applied on top of
each catalyst layer instead of the conventional steam or air soot blowers.
Fig. 7
Arrangement of DeNOx Plant for Petersburg Station Unit 2
Fig. 8
Arrangement of DeNOx plant for Petersburg Station Unit 3
(2)
Operating results of Petersburg station Unit 2 & 3 DeNOx plants
In the spring of the year 2004, the DeNOx plants for both units were got into operation. After
the initial operation adjustments, the performance tests were performed in June 2004.
The performance test results were summarized in Table 5.
Table 5
Parameter
DeNOx Efficiency
NH3 Slip
Catalyst ΔP
SO2 to SO3
Conversion Rate
Performance Test Results
Performance Test Results
Units
Guarantees
%
90
Unit 2
90
Unit 3
90
ppmvd @ 3%
O2
inch W.G.
Less than 2
<< 2
<< 2
1.4(Unit 2)
1.8(Unit 3)
Less than 0.5
1.1
1.5
< 0.3
< 0.3
%
As shown in Table 5, the catalyst installed in DeNOx plants at Petersburg station met all of
the guarantees. In particular, measured SO2 to SO3 conversion rate was considerably lower
than the guaranteed 0.5% conversion rate.
The DeNOx plants at Petersburg station have been operated successfully during the year 2004
Ozone season without any problem for both DeNOx and boiler systems.
During the annual inspection period of boilers, inside inspection of DeNOx plants were
conducted. Representative observation results were shown in Fig. 9 and 10. As shown in the
Figures, both ash accumulation and erosion of catalyst were not observed.
Fig. 9
Fig. 10
Inside Inspection Result – General View -
Inside Inspection Results – Close-up Observation of Catalyst -
(3) Other applications
The newly developed CX Type catalyst was supplied to various DeNOx plants since the first
installation at Petersburg station. Table 6 shows the experience list of CX Type catalyst that
was supplied by Spring of 2005.
Table 6
No.
Plant Name
Experience List of CX Type Catalyst
Catalyst Type
Start Operation
Note
1
Petersburg 2
CXL
2004
New Installation
2
Petersburg 3
CXL
2004
New Installation
3
Farge
CXM
2004
Replacement
4
Datteln
CXM
2004
Replacement
5
Muskingumriver
CX
2005
New Installation
6
Gavin 1
CXL
2005
Replacement
7
Keystone 1
CXM
2005
Addition
8
Trimble County 1
CXM
2005
Addition
9
Logan
CXM
2005
Replacement
10
Lambton 4
CXL
2005
Addition
11
Duernrohr 1
CXM
2005
Replacement
4. Benefits for Application of CX Type Catalyst
The design requirements for European DeNOx plants are much moderate compared with U.S.
applications as shown in Table 7.
Table 7
Requirements
DeNOx Efficiency
Slip NH3
SO2 concentration
Comparison of Design Requirements of DeNOx Plants
U.S.A.
More than 90 %
Less than 2 ppm
2,500 - 4,000 ppm
Europe
60 - 80 %
3 - 5 ppm
1,000 - 1,500 ppm
Because of these differences, operational benefits shown in Table 8 can be achieved by
utilizing the superior characteristics of CX Type catalyst as described in Paragraph 3.
Table 8
Benefits of Application of CX Type Catalyst
Existing DeNOx Plants
New Installation
-
Extension of catalyst maintenance
interval
-
Low SO2 to SO3 conversion rate
Smaller reactor
-
Reduction of SO2 to SO3 conversion rate
Reduction of pressure drop(Application
-
Lower pressure drop (Application of
wider pitch catalyst)
of wider pitch catalyst)
Apart from above benefits for the existing DeNOx plants, utilization of CX Type catalyst can
also achieve higher NOx removal efficiency which may contribute in reducing total NOx
emission from the nation.
5. Conclusion
Babcock-Hitachi developed new CX Type catalyst, having low SO2 to SO3 conversion rate
maintaining high DeNOx potential.
The catalyst was first applied to the DeNOx plants at Indianapolis Power & Light/Petersburg
station Unit 2 & 2, and achieving less than 0.3% SO2 to SO3 conversion rate even at the
elevated temperature as 415ºC.
By application of CX Type catalyst in European DeNOx plants, the catalyst can achieve
various benefits for the plant operation.